The size of the patch of wave front that can be presumed to
be flat changes depending on the seeing conditions. In the daytime with solar
heating it can be very small. The focal point of a single patch of wave front
will form a star that jiggles continually with the
turbulence. Increasing the
aperture of the telescope to include many patches of wave front produces a blur
of multiple images. Larger telescope apertures enclose more patches of wave
front requiring an adaptive correcting mirror with more elements. The
collimated light beam striking the correcting mirror is a miniature of the
collimated beam that stuck the main telescope mirror. For this reason the
optical elements in the correcting mirror are smaller than the original patches
of wave front that struck the primary mirror.

In the ionosphere there is a layer of sodium atoms.
The sodium atoms come from tiny meteors. The sodium atoms in this layer can be ionized by
use of a powerful laser producing
a tiny artificial star. The cylinder of ionization in the ionosphere is about a
meter in diameter but from the earth the wave front sensor is distant enough
from the ionization to see only a star size diffraction pattern. Ref 1